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Discussion Starter · #1 ·
Hello everyone, as the summer comes you see plenty of new projects online, a thing that became more and more common apparently is modern front fork swap on vintage bikes, what's your opinion on front fork swaps? or even better, if someone who attempted sees this post, could you give some feedback about the swap?
I have seen some swaps done even on this forum, but not many of the people who have done a swap give their opinion if it's worth it or not.
For example I have a honda cb400t (cb400n) and apparently the honda cbr 600 rr and 2007 Suzuki GSXR 1100 RR are direct swaps, I'd love to hear the feedback of someone who attempted it.
Since I know someone will link the thread of nerdsports, I've read many times his thread but there isn't much feedback of the swap, also he had a mono-swing arm so even if he answered this thread his bike would be very different from a stock one swapped with just the forks.

I suppose since most of modern fork are shorter the bike would "lean" more forward so I suppose the bike would be more front heavy having the weight of the bike itself and the biker more on the front, I suppose you could compensate this a little having the biker and foot pedals more toward the back.
I am not that experienced of a driver, but wouldn't something like this, under breaking, feel like the front wheel "dips" more in the road (I can't find a better way to express myself) and have the rear feel very light if not even lifting, you would need seriously stiff front suspension to compensate this.

I have so many questions...
 

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If your looking at better "performance" regarding front ends.. you must first understand "rake and trail"...just swapping forks may only change the "compression". There are a few variables with rake and trail, angle of neck, fork lengths, and even wheel size.. I'm not by any means an engineer or a rocket surgeon.. but this is what I discovered when I looked into making a guirder front end, and when I swapped to a "springer" style front end on my other bikes..
 

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I just ran a search for Tony Foale's book, behold someone has it hosted.

I'll post the link but I'm not sure if it'll be allowed to remain here. Mods please assist if needed. I can always pm the link to anyone interested as well.
 

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I removed the link, copyright infringement issues since this is a commercial site. Same reason we had to remove links for manuals years ago. People can PM you for the link.
Great book, some pretty heavy reading.
 

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A fork swap opens a HUGE bucket of worms. Not necessarily a bad idea, but you need to know what you're doing.
If you can find a more modern front end that simply bolts into place, you've done a bunch of the hard work (i.e. not having to re-machine the threaded stem, make bearing adapters, etc.). But that's just the start.
If you're particularly lucky, the effective fork length will stay the same- i.e. the distance from the lower triple clamp to the axle center is the same. Too long and you end up with excess rake (think old school chopper) which will slow steering response but add to straight line stability. Too short and the opposite happens- quicker steering but less stable (which can cause problems like being more prone to tank slappers, among other things).
But that's only the beginning. If the relationship between the fork tubes and the steering stem is the same between the original forks and the new forks, you've dodged a bullet. But because they were designed for two different bikes, they're likely not. If the new fork tubes sit farther forward from the stem relative to the original fork tubes, you'll get more trail, which like rake is good for stability but bad for responsiveness. And like above, if the fork tubes sit farther back relative to the stem, you get less trail and stability.
Then there's the fork tube diameter. Newer forks will almost certainly be larger in diameter than the original ones. This makes the fork assembly stiffer, which is good for responsiveness. But excessive stiffness means more loads fed into the frame, which may or may not be up to the challenge. Bad consequences involve wonky handling as the frame deflects under load, in ways you're not expecting.
Solve all that, and you're also needing to deal with spring stiffness. I'm guessing the springs for a GSXR1000 are going to be significantly stiffer than your stock CB400 springs, since the gixxer is heavier and designed as a pure sports bike. The stiffer springs will give a rougher ride. You can compensate by getting custom springs (generally under $200) that are calibrated for the weight of you and your bike. But then you have to adjust damping.
The forks have internal passages and valves that act to allow the forks to compress rather freely, but extend in a controlled manner. Change spring rate, and the damping must be adjusted to suit the new springs. This may be as simple as changing fork oil, or as complex as having the internals re-valved.
Oh, did I mention travel? If the new forks have more travel than the original ones, you run into more interesting possibilities of undesirable handling.
Now that that's all sorted, what about the wheel and brakes? Modern sport bikes have fantastic brakes, which is a good thing. But see earlier note about feeding more forces into a frame that's not designed for them.
Ditto for the front wheel. It'll likely be considerably wider than your original front wheel, and accept far grippier radial tires. Which is great for stopping and cornering grip, but bad in terms of handling (does the front tire end up being as wide if not wider than the rear tire) and, again, frame flex.
And that's just the simple version. I'm sure my comments don't cover everything. And I'm not trying to talk you out of it. Just be aware of what you're getting into.
HTH
 

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I'm far from an authority on such things, and a VF500F is far from modern, but I did put a VF500F front end on my '82 CB450SC. You can read about my experience here.
 
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I just finished a similar project, using a 97 nighthawk 750 donor. Had to swap stems, used triples, forks and wheels from 750. Fork length was addressed by mounting clip ons above upper triple. Rear swing arm is from a 450 nighthawk, axle spacers are custom. Steering stop proved challenging, did lose a little turning radius. (Only affects garage maneuvering) Haven't put a lot of miles on yet, but I like it so far. New head bearings from All Balls worked great. Also had to swap front sprocket to a offset 525, rather than stock 530. Would I do it again? Ask me in a year, or a couple thousand miles.
 

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Discussion Starter · #9 ·
I just ran a search for Tony Foale's book, behold someone has it hosted.
I actually know the book, what keeps me from reading is the language barrier, I might give it a shot tought

And I'm not trying to talk you out of it
There is no need to, time and money do a good enough job to keep me from attempting it. I was asking more out of curiosity since I see so many projects doing front ends swap and I wanted to know if the bikes I see are undrivable or if the builders put more effort in than what I originally gave them credit for

I'm far from an authority on such things, and a VF500F is far from modern, but I did put a VF500F front end on my '82 CB450SC. You can read about my experience here.
I have read your thread before, although it's pretty cool, I was refering to more modern swaps thinking of frame flex (like Old Putz mentioned). I am sure yours wasn't an easy swap but I think since the forks came from a similar period frame flex wasn't a concern

1982 Cb450t frame, Cm450e engine, Cm400t exhaust, and Cb400t tank, side covers and seat.
That's an interesting combination, if you didn't tell me I would have never spotted it's not a regular cb 400 t!
 

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I swapped fork, tree and triple, front wheel and rear wheel on my cl450 from a donor 81 cb400T hawk. Two issues there...81 hawk has better forks than those on a 72 450 but they are far from modern...just better. They also allowed me to use comstar wheels with a disk brake.

The other issue is the cb400 fork is a 33mm fork and the original 450 fork is 35mm. I wasn't pleased going to a lighter fork but the overall net was better aesthetics ( in my eye), better front brake and the fork performance was a wash...thinner cb400 about the same as original, older 450 fork.

The fork swap was "bolt on" with all balls bearing kit and cm400 tree. Rear wheel required fabrication on the cb400 spacer (same work WITwinfan described) but used the original 450 swing arm. btw..I'm not the only person who has done this swap...credit for 1st attempt goes to Bill.
 

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I have read your thread before, although it's pretty cool, I was refering to more modern swaps thinking of frame flex (like Old Putz mentioned). I am sure yours wasn't an easy swap but I think since the forks came from a similar period frame flex wasn't a concern
It actually was easy and I'd wager that being able to lift the rear wheel under hard braking would put maximum load on the front of the frame. I can't see how a more modern fork could put more load or stress than that.
 

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I believe if money isn't an issue you could order full custom stem and triple clamps that keep your original offsets and match your steering stem from cognito moto. I definitely want to try it but need to understand more about rake and trail before attempting it.
 

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Take a look at some of the docs in my signature line link, maybe they'll offer some new info and tools.
 

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Been thinking of doing a front end swap, understanding the problems that may come into play. Wouldn't mind putting a "Rebel" front end on a CM200T. The Twinstar lead to the development of the Rebel and the are similar in some small ways. Only reason for the swap would be to have a disk instead of drum brake. (Tire for the front of the Rebel is also a bit easier to come by.)
Any thoughts??
 

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Bolt it on, same bearings are used on both, if steering stems are the same length it'll be a direct replacement. Rebel forks are shorter than 250 Nighthawk forks NH are 29-1/2"
 

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I personally like the retro look of the drum brakes. These aren't really performance bikes in the first place.
20180408_121443.jpg
 
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